Three pipes P,Q and R can fill a tank in 12, 15 and 20 minutes
respectively. If pipe P is opened all the time and pipe Q and
R are opened for one hour alternatively. The tank will be full
in

Options

A. 5 hr
B. 6 hr
C. 7 hr
D. 8 hr

Answer

(c)

Explanation

`(1//12 + 1//15) + (1//12 + 1//20) = 17/60` (in 2 hrs this much)
tank is filled)
So in 6 hrs 51 /60 is filled . Remaining, ` 9/60= (1/12 +1/15 ) xx T, `
so `T = 1 hr`
so total ` = 6 + 1 = 7 hr`